This invention relates to improved heat treated rods, such as grinding rods, for use in conventional rotating grinding mills or rod mills wherein material such as ore, stone and the like is comminuted, and to a method for production of such improved grinding rods by variable quenching. More specifically, the improved grinding rod of the present invention comprises a monolithic cylindrical member of high carbon or alloy steel which is heat treated to obtain a martensitic microstructure over most of its length having a Rockwell C surface hardness greater than 50 (and a softer core region), but with relatively soft end portions having a substantially pearlitic microstructure which minimize spalling and splitting thereof and reduce breakage and wear of rod mill liners.
The prior art has disclosed cylindrical rods having soft cores with the soft cores being exposed only at each end or base of the cylinder. In all such structures of which applicants are aware, the core is of a different type of metal than the outer, harder shell. Thus, U.S. Pat. No. 1,016,272, to Johnson, discloses a rod having a core of soft or wrought iron or soft steel and a surrounding body of hard chilled iron or steel which is cast or fused around the soft core.
U.S. Pat. No. 1,398,970, to Leddell, discloses a rod comprised of a plurality of plugs "which may be of a cheap grade of steel" and an outer sleeve or tube which is a "high grade steel". The plugs are held in place frictionally within the tube by shrinking the tube upon the plugs, or by pressing or hammering the assembly on opposite sides thereof to an out of round configuration.
U.S. Pat. No. 1,661,567, to Floyd, discloses a rod-like structure having a plurality of abrasion resistant steel sleeves in alignment with axial polygonal openings extending therethrough, and a steel core having surfaces inserted through the sleeves, the core having threaded ends on which nuts are engaged to hold the sleeves in assembled relation.
In its broadest aspect the method of the present invention involves the concept of variable quenching by passing a heated, elongated metallic article in a path of travel through at least one liquid quench zone, applying a liquid quench medium to a selected segment or segments along the length of the article, and turning off the liquid quench medium when the remaining segment or segments pass through the quench zone. In the case of an elongated steel article heated above the A.sub.3 point, quenching a selected segment or segments rapidly to a temperature below the Ms point results in transformation of the austenitic microstructure to martensite with consequent increase in hardness, in known manner. To the best of applicants' knowledge the above concept has never been applied to the heat treatment of grinding rods, although the prior art has suggested the concept of variable quenching by methods different from that of the present invention, of product forms such as pipes, axle shafts and the like.
U.S. Pat. No. 2,879,192, to Gogan, discloses a method and apparatus for quenching a heated workpiece in which the workpiece is wholly submerged in the quench liquid, and a portion of the workpiece is shielded from contact with the liquid, thus maintaining a void space around the shielded portion, with a cooling air flow being directed thereagainst. The method is indicated to have particular utility for heat treatment of an axle shaft having a bolt flange since it is desirable that the metal of the flange, particularly at the junction of the flange and shaft portions, be hardened to a lesser extent in order to avoid brittleness.
U.S. Pat. No. 3,140,964, to Middlemiss, discloses a method of hardening metal pipe in which a cover plate having a hole therein is welded to one end of a pipe length, and the heated pipe is passed through a quenching zone with the covered end trailing. The vent hole in the cover plate is alleged to permit flame and hot gases to enter the interior of the pipe during heating, to permit hot air and gases to leave during quenching and to admit a controlled quantity of quench water to the interior of the pipe, thus providing a cooling rate sufficient to obtain a required metallurgical structure.
U.S. Pat. No. 3,189,490, to Scott, discloses a process and apparatus for reducing cracking of the trailing end of pipe due to quench spray entering the pipe interior before the trailing end has had extended quenching. Means is provided operable by the trailing end of each pipe section to move spray quench nozzles in the direction of travel of the pipe section when the trailing end reaches a preselected position prior to entry into a fluid spray zone. Means is also provided to return the spray nozzles to their original position after the preselected movement so that when the trailing end of the pipe section approaches the spray zone the nozzles will not spray fluid internally of the pipe until the trailing end has had extended quenching. Since the spray nozzles are angled in the direction of pipe travel, quench fluid does not enter the leading end of the pipe section.
U.S. Pat. No. 3,671,028, to Hemsath, discloses a quench system in which a barrier is provided on the front of the unit to prevent quench liquid from splashing into the open leading end of a heated pipe section which is being quenched. The barrier may be a gaseous jet stream or a shield made of heat resistant material.
It is evident that the prior art summarized above does not contemplate nor deal with the problem of heat treating grinding rods in such manner as to obtain high surface hardness but at the same time to avoid spalling and splitting of the ends thereof and to reduce breakage and wear of rod mill liners.
It is a principal object of the invention to provide an improved grinding rod and method for making it which solves the above problem.
In accordance with the present invention, a heat treated rod comprises a monolithic, elongated, cylindrical high carbon or alloy steel member the end portions of which have a hardness characteristic of a substantially pearlitic microstructure, the remainder of the member intermediate the end portions comprising an annular outer region and a core region, at least the outer region, which has a surface hardness greater than 50 on the Rockwell C scale, being a substantially fully martensitic microstructure. When the rod is intended for use in a grinding mill, the core region has a hardness characteristic of a pearlitic microstructure.
In a preferred embodiment wherein a grinding rod is fabricated from high carbon steel, the hardness of the end portions ranges from about 35 to about 50 on the Rockwell C scale, the surface hardness of the annular outer region ranges from about 55 to about 60 on the Rockwell C scale, and the hardness of the core region ranges from about 30 to about 45 on the Rockwell C scale. The end portions include the entire base surfaces of the cylindrical member and the regions immediately adjacent the base surfaces which merge gradually into the annular outer region.
In its broadest aspect the invention provides a method for variable quenching of an elongated metal article, comprising the steps of heating the article to a desired temperature, passing the article in a linear path of travel through at least one liquid quench zone, detecting the position of the leading end of the article prior to entering the quench zone, initiating a liquid quench spray in the quench zone in response to said step of detecting the position of said leading end after a predetermined length of the article has passed into the quench zone, turning off the liquid quench spray in the quench zone after a further predetermined length of the article has passed into the quench zone, repeating said initiating step in any subsequent liquid quench zone after said predetermined length of said article has passed into each said zone, and repeating said turning off step in each subsequent zone after said further predetermined length of the article has passed into each zone.
A method of producing rods, in accordance with the invention, comprises the steps of providing a monolithic, high carbon or alloy steel elongated cylinder, heating the cylinder above the A.sub.3 point, passing the cylinder in a linear path of travel at a predetermined speed through a plurality of successive, axially aligned water quench zones, initiating a water spray in the first of said quench zones after the leading end of the cylinder has emerged therefrom, turning off the water spray in the first quench zone before entry of the trailing end of the cylinder thereinto, repeating said initiating step in each subsequent water quench zone after said leading end of said cylinder has emerged from each said zone, repeating said turning off step in each subsequent water quench zone before said trailing end of said cylinder has entered each zone, detecting the position of the leading end of the cylinder prior to entering the first of said water quench zones, and initiating said water spray in each of said quench zones after a predetermined linear length of travel of the cylinder responsive to said step of detecting the position of the leading end.